1. Field of the Invention
The invention relates to plasma etching of shallow recesses, and particularly to precise endpoint determination for stopping the etching at the desired recess depth.
2. Discussion of the Related Art
Poly recess on oxide processing is a technique wherein etching of the recess to an accurate depth is desired. Recess structures are first etched typically into an oxide or a nitride layer, or both, which may be over a wafer or a thick layer of poly. The etch typically does not invade the substrate as is typically the case for deep trench etching processes. Next, an oxide liner may be grown within the recess. A layer of poly is then deposited onto the oxide to both fill the recesses and to form a planar layer of poly over the oxide. A subsequent etch then first strips away the planar poly. Then, both the oxide and the poly filling the recesses is etched. It is desired to have a technique for accurately etching a shallow recess to a precise depth.
FIG. 1 schematically shows a collimator section 39 of a monitoring apparatus of an etch treatment reaction chamber 22, which is described in U.S. Pat. No. 5,807,761, which is hereby incorporated by reference into the present application. The reaction chamber 22 is provided with a top view port 26 disposed above a wafer 24 and in parallel relationship therewith. When plasma processing is performed using the chamber 22 of the ""761 application, a light beam having a specified wavelength L is applied to the wafer 24 through the view port 26 via an optical cable 30 and a lens 31. This lens 31 produces a parallel light beam, which illuminates a relatively large area of the wafer 24 at a substantially normal angle of incidence. The reflected beam is focused by the lens 31 and transported via another optical cable 32 to a spectrometer (not shown) tuned on this wavelength L.
The analog signal that is output from this spectrometer is illustrative of the interferences of the reflected light. The depth of a recess etched into the wafer 24 may be determined based on the measured interference pattern.
In the ""761 patent, a digitized optical signal S is filtered to reveal two components: S1 that is illustrative of the re-deposition of etched material outside the trench and S2 that is illustrative of the trench etching. The signal S can be separated into two frequency components S1 and S2 because the trench depth is much larger than the wavelength of light and the signal S includes many periods of the interferometric oscillations.
The technique described in the ""761 patent relates to a method of real time and in-situ monitoring of deep trench depths. That is, the deep trenches of the ""761 patent have depths much larger than the wavelength of the monitoring light source (xcex less than  less than trench depth). Moreover, these deep trenches have diameters far larger than the diameters of shallow poly recesses.
Depths of deep trenches may vary by a larger absolute depth variance than would be tolerable for a shallow recess. For example, if a depth tolerance of 6% is desired for both the deep trenches (xcx9c10 xcexcm) of the ""761 patent and for shallower poly recesses (xcx9c600 nm), then the deep trenches may be etched to depths between 9.4 xcexcm and 10.6 xcexcm, or somewhere in a 1.2 xcexcm range, whereas the shallow recesses may be etched to depths between the narrower range of 564 nm to 636 nm, or a rnage of only 72 nm. Thus, in order for each process to achieve depths within the desired tolerance of, e.g., 6%, the shallow recesses must be etched with an absolute accuracy of depth around 15-20 times greater than for the deep trenches. It is desired to have a technique for achieving this higher degree of absolute shallow recess etch depth accuracy.
It is therefore an object of the invention to have a technique for accurately etching a shallow recess to a precise depth. It is a further object to have a technique for achieving a much higher degree of absolute shallow recess etch depth accuracy than is needed for deep trench etching.
In accord with the above objects, a method is provided for plasma etching a shallow recess at a precise depth by illuminating a wafer with a light source and using a spectrometer to receive the light reflected from the wafer. The method begins with a step of detecting the etch start time, preferably by detecting the time of plasma ignition out of measured reflectance data. Alternatively, a step of detecting the time when the wafer reflectance signal begins to change will be performed when etching is not started at the time of plasma ignition, e.g., when an oxide layer is etched by the plasma prior to etching the recess. The precise etch start time is preferably extracted from the reflectance data which exhibits a sharp rise at the time of plasma ignition, and then again after breaking through any residual oxide.
The next step is measuring the reflectance intensity of light reflected from the wafer. Preferably the plasma background signal is removed from this measurement and an array detector is used wherein the wavelength is determined using software analysis.
Next, a step of determining the etch rate is performed preferably by fitting data representing the collected reflectance signal to a wafer reflectance model as a function of time, and extracting the etch rate from the model. The model preferably takes into account a weakening of the reflectance signal as the recess becomes deeper, and preferably as well any etching of the mask or top reference layer.
Last, an etch stop time is determined based on the etch rate, the etch start time and the predetermined etch depth. Preferably, a software timer will then trigger the endpoint such that etching is stopped at the etch stop time.
A breakthrough step may be performed wherein a residual layer such as an oxide layer is first etched away prior to beginning the recess etching process. The software would take into account the etching away of this residual layer or layers above the recess, such that an etch start time would begin after plasma ignition and the etching away of the residual layer.
Preferably, an array spectrometer is used for detecting the reflected light from the wafer. Also preferably, wavelength selection is performed in the analysis software. Also preferably, a substantially exponential reduction in reflectance intensity based on the depth of the recess is taken into account in the reflectance model used in the analysis software. Also preferably, a chamber window is used that is not exactly parallel to the wafer surface so that light reflected from the window is not collected.